Vigorous vertical mixing supports biological production in the mesoscale Tsugaru Gyre

Abstract

Abstract Examining localized nutrient supplies that facilitate broader-scale biological production is imperative for comprehending marine ecosystems. Nutrients transported vertically in specific hotspot areas that reach the surface can be conveyed downstream by background currents, thereby augmenting surface biological production over a wider expanse, including robust currents traversing seamounts. However, assessing the influence of such transport on large-scale production poses challenges due to spatiotemporal disparities. We present an analysis of the Tsugaru Strait, a critical conduit linking the Sea of Japan and the Pacific Ocean. Our investigation utilizes high-resolution surface current data acquired from a long-term high-frequency radar system aligned with direct turbulence measurements. Shipboard observations unveil significant turbulence and diffusive nitrate fluxes, surpassing open ocean levels by a factor of 10 or more. The intensified turbulence was associated with heightened surface density, indicative of a surface divergence field. A six-year statistical analysis establishes a positive correlation between this surface divergence and downstream chlorophyll-a distribution, aligning with the formation area of the hundred-kilometre-scale anticyclonic Tsugaru Gyre. Therefore, the Tsugaru Strait emerges as a hotspot for the production of the gyre, which is renowned as a valuable fishing ground. This discovery carries significant implications for monitoring and conserving marine ecosystems to ensure sustainability.